This application claims the priority of German priority document 198 35 979.9, filed Aug. 8, 1998 and PCT International Patent Application No. PCT/EP99/05689, filed Aug. 6, 1999, the disclosures of which is expressly incorporated by reference herein.
The invention relates to a method for monitoring traffic states in a road traffic system and to a method for controlling vehicle inflow as a function of the traffic states.
Various methods of this type are known in the field of traffic control technology. The traffic states are sensed at a given time for a particular monitoring point of the road traffic system, using measuring equipment with appropriate sensors. Alternatively, or in addition, the traffic states at the monitoring point are predicted in advance. An appropriately configured traffic control computer which is normally used for this purpose, suitably evaluates the measured data, and preferably also empirically determined predicted values for the traffic states to be expected at the particular monitoring point at a time in question. The traffic state information which is determined in this way can then be used for various purposes; for example for travel time prediction, for dynamic route planning and for traffic-controlling intervention such as controlling the vehicle inflow at entries into a respective section of the traffic system. The term xe2x80x9ccontrolxe2x80x9d is used above, for the sake of simplicity, in its wider sense which includes both open-loop and closed-loop control systems.
Investigations have shown that the traffic states in road traffic systems can be divided into three significantly different types; specifically free traffic flow, synchronized traffic flow and wide moving traffic jams. See B. S. Kerner and H. Rehborn, Experimental features and characteristics of traffic jams, Phys. Rev. E, Vol. 53, page 1297, 1996 and B. S. Kerner and H. Rehborn, Experimental properties of complexity in traffic flow, Phys. Rev. E, Vol. 53, page R4275, 1996. Free traffic flow is understood here to be the state in which any road user can freely select its velocity and any desired overtaking maneuvers are possible. The wide moving traffic jams state signifies stationary vehicles with maximum traffic density on the road. Synchronized traffic flow, also referred to as stop-and-go traffic, constitutes a traffic state between free traffic flow and the wide moving traffic jams state, in which the traffic density (i.e., the traffic flow) may be relatively large but is significantly higher, and thus the velocity of the vehicles is significantly lower, than in free traffic flow, which very greatly increases the trip time. Owing to the higher traffic density, overtaking maneuvers are virtually impossible; for this reason the velocity of the vehicles at one location on the different lanes of a multilane road (expressway) is slow-moving when all the lanes are going in the same direction.
There are numerous known methods for detecting the wide moving traffic jams state by analyzing locally measured traffic data, including disruption detection and analysis. See, for example, German patent document DE 196 47 127 A1 and the literature referred to in it.
The control of the inflow of vehicles, also referred to as inflow metering, constitutes one of the possible ways of controlling the traffic flow when traffic disruption is detected or predicted, and thus preventing the occurrence of disruption or in any case as far as possible restricting its consequences in order to minimize increasing trip times and to maximize the efficiency of the roads. There are numerous known methods for inflow metering for entries to expressways. For example, a simple strategy frequently used in the USA has been to simply close off the entries when traffic comes to a standstill; but methods have also been used there in which the total of the inflow and upstream measurement in comparison with the downstream capacity of the road has been used as a criterion for restricting inflow, see L. E. Lipp, L. J. Corcoran, A. H. Hickman, Benefits of central computer control for Denver ramp-metering system, Transportation Res. Board No. 1320, Washington D.C., 1991 and N. L. Nihan, M. G. H. Bell, A predictive algorithm for real-time ramp control system, ITE Journal, June 1992.
In Great Britain a multilayered algorithm has been used for inflow control. In this algorithm the road capacities were monitored and an inflow control was carried out at excessively low velocities, the space/time profile of the waves of traffic density being tracked and the queue length of backed-up vehicles being used. See D. Owens, M. J. Schofield, Access control on the M6 motorway: evaluation of Britain""s first ramp-metering scheme, Traffic Engineering+Control, page 616, 1988. In the Netherlands, on the other hand, a concept of individual metering for vehicle circulation times between 4.5 seconds and 12 seconds with the latter value as the maximum possible value was investigated. This concept corresponds to metering between 300 vehicles/h and 800 vehicles/h. See H. Bujin, F. Midelham, Ramp metering control in the Netherlands, Road Traffic Control May 1990 and Projektbericht DRIVE I Project V 1035 CHRISTIANExe2x80x94Isolated Ramp Metering: Real Life Study in The Netherlands, Deliverable 7a, March 1991 of the EU project CHRISTIANE.
In France, CHRISTIANE was developed and used within the EU project and subsequently the ALINEA algorithm was developed and used in field trials in Germany (and in Germany in a modified form with the traffic density instead of the degree of occupancy). See the project report DRIVE I Project V 1035 CHRISTIANExe2x80x94Isolated Ramp Metering: Real Life Study in France and Software Prototypes, Deliverable 7b, October 1991 and P. Stxc3x6veken, Verfahren zur Steuerung des Verkehrsablaufs auf Stadtautobahnen mittels Geschwindigkeits- und Zufluxcex2regelung, Straxcex2enverkehrstechnik June 1992.
Both the wide moving traffic jams state and the state of synchronized traffic flow are highly significant for maintaining the greatest possible efficiency of road use. The trip times in the case of synchronized traffic flow are significantly increased in comparison with free traffic flow, which is undesirable in itself, and in addition for associated applications, for example telematics applications. There is therefore a need for a method for detecting reliably the state of synchronized traffic flow and distinguishing in particular from the state of free traffic flow so that this information can then be suitably used for inflow metering which exploits the efficiency of the road in the best possible advantage and/or for short-term prediction of trip times.
One object of the invention is to provide a method for monitoring traffic states of the type described above.
Another object of the invention is to provide a vehicle inflow control method which uses such monitoring method and with which the traffic transitions can be reliably monitored, and when necessary estimated in advance, in particular with regard to phase transitions between free traffic flow and synchronized traffic flow and/or with regard to wide moving traffic jams states.
Finally still another object of the invention is to provide a method and apparatus which achieves a high degree of efficiency of a monitored section of the traffic system, with relatively little expenditure.
These and other objects and advantages are achieved by the method and apparatus according to the invention, in which current or predicted traffic states are determined for one or more points and a distinction is made between the three types of traffic states: free traffic flow, synchronized traffic flow and wide moving traffic jams. Vehicle inflow into the traffic system is then controlled as a function of the detected traffic states. The state monitoring method is configured to detect or predict phase transitions between free traffic flow and synchronized traffic flow and/or wide moving traffic jams states, by means of specified criteria. Furthermore, according to the invention the vehicle inflow into the monitored traffic system section is controlled as a function of detected phase transitions between free traffic flow and synchronized traffic flow.
The monitoring methods according to the invention permit comparatively reliable detection of the phase transitions from free traffic flow to synchronized traffic flow and vice versa from synchronized traffic flow to free traffic flow, using relatively simple means. The conditions utilized for this purpose both provide a reliable way of distinguishing between free traffic flow and synchronized traffic flow and can be tested using measuring and computational equipment with an acceptable degree of expenditure.
The measured parameters which are used for this, such as the average velocity, (i.e., the average velocity of vehicles passing the monitoring point on one or more lanes of the road), and the traffic flow, (i.e., the number of vehicles passing the monitoring point per time unit) can be sensed easily. Traffic flow is to be understood here and below in all cases as a traffic flow per lane; that is, either for each lane or averaged over all the lanes of one roadway. Accordingly, inflows and outflows are always related to the respective number n of lanes, i.e., divided by n.
The high level of significance particularly of the phase transition from free traffic flow to synchronized traffic flow in terms of ensuring the maximum possible efficiency of the road and in terms of predicting traffic is due particularly to the fact that in synchronized traffic flow the throughput rate of vehicles can be virtually the same as for free traffic flow despite the very greatly increased trip time. The detection of the phase transition to synchronized traffic flow, and the dispersal of such traffic state and a return to the state of free traffic flow makes it possible to take suitable countermeasures in good time when synchronized traffic flow occurs. These phase transitions can be determined as an anticipated phase transition, both for the current time and, when necessary, also as part of a prediction relating to the future traffic states.
In the method according to one embodiment of the invention, in order to detect a phase transition to synchronized traffic flow, the average velocity and the traffic flow are tested to i) determine whether the average velocity decreases to a greater extent than a predefined amount, and ii) whether the traffic flow is more than a predefinable flow threshold value. The former condition makes use of the observation that at the transition from free traffic flow to synchronized traffic flow the average velocity decreases comparatively quickly. With the second condition, the state of synchronized traffic flow is distinguished from the wide moving traffic jams state since in the latter the traffic flow is significantly lower than in the case of synchronized traffic flow.
According to another embodiment of the invention, in order to detect a phase transition to synchronized traffic flow specific interrogations are made regarding the conditions as to whether: i) the average velocity is decreasing ii) the traffic flow is more than a predefinable flow threshold value, and iii) the quotient formed from the change in the average velocity divided by the change in the traffic flow exceeds a predefinable threshold value in absolute terms. The first condition makes use of the observation that at the transition from free traffic flow to synchronized traffic flow, the average velocity decreases comparatively quickly and significantly, whereas the traffic flow does not exhibit such a severe change.
According to another feature of the invention, future phase transitions from free traffic flow to synchronized traffic flow are estimated in advance as part of a traffic prediction; that is, an advance calculation of the expected traffic states in the road traffic system (and/or specific sections thereof). Such phase transitions are caused by upstream phase transitions which are detected at the given moment. This detection in advance of future states of synchronized traffic flow can be advantageously used to improve the estimation of anticipated trip times and to initiate, at an early point, suitable countermeasures with which an expected slowdown of the traffic (or even wide moving traffic jams) can be counteracted by means of appropriate traffic control measures. The criterion which is used for the prediction also takes into account the case in which entries and/or exits are located between the point of the currently detected traffic states and the point of the predicted upstream synchronized traffic flow state.
In a monitoring method developed according to another embodiment of the invention, the duration of a synchronized traffic flow state which has been caused by a currently detected phase transition from free traffic flow to synchronized traffic flow upstream of an entry or exit is estimated in advance by means of specified criteria. Entry is to be understood in this case in the broader sense, to include a constriction at which the number of lanes is reduced. In an analogous way, in another embodiment the spatial extent of such an induced, synchronized traffic flow state is predicted on the basis of specified criteria.
In still another embodiment of the method according to the invention, a phase transition from synchronized traffic flow to free traffic flow is deduced if the average velocity exceeds a predefinable velocity threshold value or rises above a predefinable velocity value by more than a predefinable degree. The state of synchronized traffic flow is not dispersed, and thus a transition to free traffic flow is not achieved until, due to an appropriate hysteresis phenomenon, there are significantly lower traffic densities than the inverse situation, when synchronized traffic flow is formed from previously free traffic flow. It therefore becomes apparent that the inventive observation of the average velocity to determine whether it exceeds a certain threshold value or rises above a predefinable velocity value by more than a predefinable degree constitutes a very reliable criterion as to whether the state of synchronized traffic flow has been dispersed and has changed into free traffic flow.
The monitoring method according to yet another embodiment of the invention, constitutes an improvement of the method described in German patent document DE 196 47 127 A1 (referred to previously) and permits a comparatively reliable estimation in advance of the development of a predicted wide moving traffic jam state which is occurring at a given moment, has been detected or will occur in future. This prediction of the development of wide moving traffic jams states can then be taken into account, for example, in a trip time prediction. It is apparent that with this method the start of the wide moving traffic jams state and the end of the wide moving traffic jams state, and consequently all the aspects of the development of the wide moving traffic jams can be forecast comparatively reliably.
According to another feature of the invention, it is possible to estimate in advance the velocity values of the upstream and/or downstream front wide moving traffic jams from previously available traffic state data for a future period if no more recently updated traffic state data can be acquired in this period. The future positions of the upstream and/or downstream front wide moving traffic jams can then also correspondingly be determined.
An inflow control method according to the invention makes use of the observation of phase transitions between free traffic flow and synchronized traffic flow by means of traffic state monitoring as described above, in order to appropriately control the vehicle inflow at a respective inflow point, as a function of these phase transitions. This use of detected phase transitions from free traffic flow to synchronized traffic flow as a basis of an inflow control system helps to optimize the traffic flow in the road system, without the need of frequent control interventions into the traffic flow. This low frequency of control interventions into the traffic inflow advantageously also ensures that their effects on the secondary traffic system sections from which the inflow takes place is kept low. Overall, in this way the inflow control method according to the invention under the given conditions of a continuously growing traffic volume ensures optimum efficiency of the traffic system, in particular on expressway sections thereof.
In another embodiment of the inflow control method herein, inflow is restricted if a phase transition from free traffic flow to synchronized traffic flow is detected at a monitoring point which is nearest in the downstream and/or upstream direction to the inflow point.
Finally, in another embodiment of the invention, the inflow restriction which is activated beforehand at the transition to synchronized traffic flow is lifted again if a phase transition to free traffic flow is detected at the nearest upstream and/or downstream monitoring point; that is, the previously detected synchronized traffic flow has dispersed again to form free traffic flow.